1. Field of the Invention
The present invention relates to an antenna device having a carrier structure and a matching means. In particular, the invention concerns an antenna device for a mobile personal telephone. More specifically, the invention is directed towards an antenna device with at least one radiating element having half-wave characteristics.
2. Description of the Related Art
One way of making use of several advantages of the invention, is applying it to an antenna device having a fixed helical radiator in combination with an extendable and retractable straight radiator, at least one being essentially of half-wave type.
In a radio communication device the transmitting/receiving circuits are coupled to the feed point of a radiator means via a feed line. Normally, the transmitting/receiving circuits have a nominal impedance of 50+j0 ohms. If the impedance of the radiator feed point differs substantially from that of the transmitting/receiving circuits, an impedance matching means is required for matching the impedance of the radiator to the impedance of transmitting/receiving circuits.
The radiator and the impedance matching means may then be interconnected by a first feed line or similar having a given length. The first feed line is influenced electromagnetically by different conductive or dielectric bodies in its surroundings, for example, a support on which the radio device is resting, the hand and head of an operator, or the chassis of the radio device or conductive parts thereof. Especially when the radiator feed point impedance is high, the length of the first feed line is significant to the environmental influence on the antenna performance. The higher the impedance of the feed point and the longer the first feed line, the higher the sensitivity to environmental variations.
A radiator of quarter-wave type may not require an impedance matching means to be connected to 50 ohm circuitry. Sometimes, a quarter-wave radiator is preferred since it allows the antenna means to be relatively short and non-obstructive. However, a drawback of quarter-wave radiators, for example in cellular telephones, is that currents are inevitably generated on the chassis of the telephone. The antenna performance is then sensitive to influence by, for example, the operator holding the telephone or pressing it to his ear.
Also, from another point of view, it is desirable to use a radiator with relatively high impedance, for example a half-wave radiator or similar. Generally, a half-wave radiator provides a higher efficiency and a greater overall length resulting in less screening. Particularly, on a small size cellular telephone, screening by the operator's head is a problem with regard to operating range and absorption of radiation in the human body.
An antenna device called RA 3137, designed by Allgon Antenn AB in 1993, includes a housing carrying a straight essentially half-wave radiator, a coaxial connector, and an impedance matching means connecting the radiator and the connector. The impedance matching means consists mainly of a coil located inside a cylindrical ground portion, and interconnecting the radiator feed point and the center pin of the connector. Both the coil and the connector are expensive components in manufacture and assembly. RA 3137 is considered to be the prior art closest to the invention.
U.S. Pat. No. 5,300,940 discloses a similar antenna device for a mobile radio communication device. That antenna device includes a helical or straight radiator and at least one reactive element in the form of, for example, a coil on a core for adapting the radiator impedance and for obtaining a desired bandwidth.
The two above mentioned antenna devices fail to be cost-effective, compact, and provide ease of manufacture in large quantities.
WO 94/10720 discloses an antenna device which comprises a housing carrying a helical radiator and an extendable straight half-wave type radiator, which are connected to an impedance matching means inside a portable telephone. The contents of WO 94/10720 are incorporated herein by reference.
Apparently, problems related to the use of a half-wave type radiator in a mobile radio communication device (a telephone) are: the presence of a high-impedance feed line which is sensitive to disturbance and the need of an impedance matching network which will excessively occupy space inside the telephone or the antenna base.